1. Field of the Invention
The present invention relates to field of mounting cylindrical embossing tools for embossing rolls.
2. Description of the Background Art
This invention relates to a mounting cylinder and a method for mounting cylindrical embossing tools for embossing rolls, in particular for embossing diffraction gratings or holograms on plastic foils and other substrates. Such embossing tools have a partially or completely structured surface which, after the embossing tool has been mounted on the mounting cylinder, is molded into an embossing lacquer by the endless rotary method. Because the lacquer layer into which embossing is done is relatively thin, it is preferable to use “endless” embossing tools that do not have either an abutting edge or butt joint or a bulky seam.
Mounting cylinders for similar purposes are known from printing technology. However, the printing forms used here that are mounted on the mounting cylinder are thick-walled and very massive, so that the mounting cylinder is accordingly adapted mechanically due to the high clamping powers occurring during mounting. Printing technology uses for example very flexible printing forms made of rubbery materials which require different mounting mechanisms. Some printing processes also use thin metallic plates as printing forms, which are mounted e.g. magnetically and in abutting relationship. Mounting cylinders from printing technology are therefore unsuitable for embossing diffraction gratings and holograms, since the embossing tools used for embossing, which are usually made of nickel, exist as tubular, thin-walled so-called sleeves with a wall thickness of only a few tenths of a millimeter. Mounting the thin-walled sleeve on the mounting cylinder can be done according to different principles.
According to one principle, the mounting cylinder or at least its outer, cylindrical mounting shell is cooled from inside, for example by continuous flushing with liquid nitrogen. The mounting cylinder or mounting shell is made of material with a high heat expansion coefficient, for example aluminum, so that its outside diameter shrinks accordingly due to cooling. In this state the cylindrical sleeve can be easily drawn over the mounting cylinder or shell. After the end of nitrogen cooling the diameter expands back to its original value due to heating. This causes the surface of the mounting cylinder or shell to come in direct contact with the sleeve and tightly mount it. This principle is mentioned for example in DE 100 49 283 A1.
According to another principle, the sleeve is expanded elastically by compressed air, an air cushion being created between the mounting cylinder surface and the sleeve via which the sleeve can be easily slipped onto the mounting cylinder and removed therefrom. Switching off the compressed air causes the sleeve to contract radially and come to rest on the surface of the mounting cylinder, thereby being firmly mounted on the surface. This principle is also mentioned in DE 100 49 283 A1.
In connection with the aforementioned principles for mounting a sleeve, for example using compressed air, DE 101 02 269 A1 describes a method for mounting and dismantling a shell without the whole carrier roll having to be removed from the plant. Accordingly, a bearing of the cylinder shaft is merely swung out so that the shell can be slipped onto a carrier core of the mounting cylinder or removed from said core over the swung out, free end of the shaft.
While forms for printing technology are usually not tempered during the printing process, tempering of embossing tools is of essential importance for embossing diffraction gratings and holograms in order to achieve good transfer and good embossing results when embossing the optically diffractive structures. If embossing is done in radiation-crosslinking lacquers for example, it can be necessary to remove polymerization heat to avoid overheating of the substrate. In the case of heat curing lacquers, however, the supply of heat can be required. Therefore, adequate tempering of the sleeve is of particular importance for embossing optically diffractive structures.
For heat curing systems, DE 100 39 744 A1 proposes instead of an external supply of heat an internal tempering of the embossing tool (“sleeve”) by which during the embossing process the embossing tool is brought from inside to a temperature determined as optimal. The mounting cylinder described therein comprises a hollow shaft, a mounting shell with an outer surface for mounting the cylindrical embossing tool and two holding devices on the faces for fixing the mounting shell coaxially on the shaft. The mounting shell is tempered from inside by a tempering medium, in particular water, which is passed through a first channel of the hollow shaft into a hollow space between the shaft and the mounting shell and passed out of the hollow space again through a second channel disposed coaxially with the first channel in the hollow shaft. This causes the mounting shell and simultaneously also the embossing tool mounted on the shell to be tempered from inside.
In addition to the connections for the tempering medium at one end of the shaft, a compressed air inlet is integrated into the shaft at the other end of the shaft. Compressed air is branched in the radial direction by the holding device on the face and passed on through pipes extending in the axial direction in the hollow space between the shaft and the mounting shell to different through bores extending radially through the mounting shell. The compressed air exits through said bores at different places on the surface of the mounting shell, creating a compressed air cushion by which a embossing tool to be mounted is expanded radially and on which the embossing tool can be easily slipped over the mounting shell and removed therefrom.
However, the above-described mounting cylinder is of extremely complex structure. When the embossing plant is reset to a embossing tool with a different diameter in case of a format change, a whole new mounting cylinder is required. This is not only cost-intensive but also time-consuming, since it is in each case necessary to completely remove the shaft axle, thereby also separating the shaft axle from the connections for compressed air and tempering medium.